Method of operating an aluminium oxide reduction cell

ABSTRACT

A METHOD OF OPERATING A CELL OF AT LEAST 50 KA. FOR THE PRODUCTION OF ALUMINUM, COMPRISING CHOOSING THAT ANODIC CURRENT DENSITY WITH WHICH, WITH A DETERMINED ELECTROLYTE TEMPERATURE, A DETERMINED INTERPOLAR DISTANCE AND A DETERMINED THICKNESS OF THE ALUMINA COVERING ON THE ENCRUSTED SURFACE OF THE BATH, AS MUCH HEAT IS PRODUCED IN THE CELL AS THE CELL CAN CARRY AWAY AS LOSSES, AFTER DEDUCTION OF THE USEFUL AMOUNTS OF HEAT FOR THE DECOMPOSITION OF THE ALUMINA AND FOR HEATING OF THE RAW MATERIAL.

P 1973 w. SCHMIDTHATTING ET 3,756,929

METHOD OF OPERATING AN ALUMINUM OXIDE REDUCTION CELL Filed Nov. so, 1971' 2 Sheets-Shet 1 Fig. 1

p 1973 w. SCHMIDT'HATTING ETAL 3,755,929

METHOD 01 OPERATING AN ALUMINUM OXIDE REDUCTION CELL Filed Nov. 30, 1971 4 v I 2 Sheets-Sheet :3

- usuap guauno ogpouv United States Patent ice 3,756,929 METHOD OF OPERATING AN ALUMINUM OXIDE REDUCTION CELL Wolfgang Schmidt-Hatting, Chippis, Rudolf Pawlek, Sierre, and Rudolf Taufenecker, Therwil, Switzerland, assignors to Swiss Aluminium Ltd., Chippis, Switzerland Filed Nov. 30, 1971, Ser. No. 203,383 Claims priority, application Switzerland, Dec. 1, 1970, 17,763/ 70 Int. Cl. C22d 3/12 US. Cl. 204-67 3 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION For the production of aluminium by electrolysis of aluminium oxide (A1 0 alumina), the latter is dissolved in a fluoride melt. The electrolysis takes place in a temperature range of about 940 to 975 C. The aluminium separated at the cathode collects beneath the fluoride melt on the bottom of the cell. Anodes of amorphous carbon dip into the melt from above. Oxygen is liberated at the anodes by the electrolytic decomposition of the aluminium oxide, and combines with the carbon of the anodes to form CO and C0 The principle of an aluminium electrolysis cell appears from FIG. 1 which is schematic and not to scale and which shows a longitudinal section. The fluoride melt (the electrolyte) is in a steel shell 12 lined with carbon 11, and provided with a thermal insulation 13 of heat-resistant, heat-insulating lining material. The aluminium 14 separated at the cathode collects on the bottom 15 of the cell. The surface 16 of the fluid aluminium forms the cathode. Into the carbon lining 11 there are inserted iron electrode bars 17, which carry the current outwards from the bottom of the cell. Anodes 18 of amorphous carbon dip into the fluoride melt 10 from above, and feed the direct current into the electrolyte. They are fixedly connected via conductor bars 19 and by clamps 20 with the anode beam 21. The electrolyte 10 is covered with a crust 22 of solidified melt, and a layer of aluminium oxide 23 lying above it. The distance d from the anode lower face 24 to the surface of the aluminium 16, also known as interpolar distance, can be varied by raising or lowering the anode beam 21 with the help of jacks 25, which are mounted on pillars 26. By reason of the attack by the oxygen released during electrolysis, these anodes are consumed at their lower face by about 1.5 to 2 cms. daily according to the type of cell.

The anodic current density of a cell can not be chosen arbitrarily.

Between anodes and cathode the interpolar distance should not go below 4 cms., because otherwise short circuits between metal and anode can occur by reason of electromagnetic forces. Moreover the electrical eificiency (relationship of the amount of aluminium produced to the theoretical amount which could be produced according to Faradays law) is low if the interpolar distance is too small.

Patented Sept. 4, 1973 On the other hand with too great an interpolar distance an unnecessary amount of heat is produced inside the cell pot, which must be carried away as Waste heat, so that the specific electrical consumption of energy (kwh./kg. Al) is unnecessarily raised.

That density of current must be chosen which produces only so much heat in the electrolyte and in the bottom of the cell, i.e. within the cell bath, as after deduction of the useful energy (that for the decomposition of the aluminium oxide and for heating the raw material to a working temperature of 940 to 975 C. with a suitable covering of aluminium oxide 23 on the solid electrolyte crust) can still be removed. The aluminium oxide covering 23 has several functions. In addition to the function of preparing aluminium oxide for introduction into the molten electrolyte, it must on the one hand protect the anodes from burning away in the air, and on the other hand form a good thermal insulation.

The least thickness of aluminium oxide cover 23 on the encrusted surface 22 of the bath can be stated to be about 7 cm. That is the minimum in operational practice.

Fundamentally one can work with a higher current density than the optimum. The excessive heat produced must then be removed by an artificial increase in the heat losses of the cell, for example by reduction of the aluminium oxide covering 23 on the encrusted upper surface of the bath 22 to 8 to 7 cms., while the specific electrical energy consumption is significantly increased. The advantage lies in this, that the cell can be made smaller, which leads to a reduction in capital costs.

If on the other hand one chooses too small an anode current density, the voltage drop in the electrolyte for constant interpolar distance falls, while the specific electrical energy consumption reduces. Since the anode current density is too small, a larger cell must be used, which is heavier and thus dearer. With increased weight of the cell, the entire building foundation construction becomes more expensive. Moreover the cost of repair increase with increasing weight of cell. The additional expenditure due to the larger cell is not compensated by the lower energy consumption. The applicants have set themselves the task of seeking Ways to the choice of the correct anodic current density for operating an aluminium electrolysis cell.

SUMMARY OF THE INVENTION The invention relates to a method of operating a cell of at least 50 ka. for the production of aluminium by electrolysis of aluminium oxide in a molten fluoride bath with pre-baked anodes. With smaller cells there is never developed too much heat.

The method comprises operating the cell with that anodic current density with which, with an electrolyte temperature between 940 and 975 C., with an interpolar distance of 5 to 6 cms. and with an aluminium oxide covering of about 14 to 16 cms. thickness on the encrusted surface of the bath, as much heat is produced in the cell as the cell can carry away as losses after deduction of the useful amounts of heat for the decomposition of the aluminium oxide and for heating of the raw materials.

FIG. 2 shows the relationship between anodic current density, 1' in a./cm. and the cell current I in ka. for the conditions mentioned. It will be recognised that the anodic current density falls with increasing cell current because the area of the cell in plan must increase more rapidly than the cell current in order to provide sufiicient peripheral wall for the heat dissipation. In the same FIG. 2 the specific electrical energy consumption E in kWh/kg. Al is marked, which corresponds to the relevant current density and the associated cell current.

From FIG. 2 that anodic current density can be read, which should be provided for according to the invention in relation to the cell current. For example, in a cell which is driven by 100 ka., the current density to be chosen is at 0.67 a./cm.

With maintenance of conditions according to the invention, the cell operates in the best range of current density, that is to say with a minimum of production costs. On the crusted upper surface of the bath there lies so much aluminium oxide in a layer about 14 to 16 cms. thick that, on the next breaking up of crust, the electrolyte can be supplied with a suflicient quantity of this preheated material. Since the surface of the aluminium oxide covering extends practically flat, an aluminium oxide layer of about 7 to 8 cms. thickness lies for example on each anode which has passed about half of its insertion time, which protects it from combustion with air. The newer anodes, the upper parts of which extend more out of the aluminium oxide covering, are only at a temperature of at the most about 500 C., and are scarcely exposed to burning away in air and require no aluminum oxide covering as protection against the atmospheric oxygen.

The interpolar distance is not too small, so that no disturbing magnetic effects can occur; it is also not so high, that unnecessary heat is produced in the electrolyte, that must be carried away out of the cell through artificially increased heat losses. The electrolyte temperature again lies in the optimum range (940 to 975 C.), so that both a good current efiiciency in cells operated according to the invention, and at the same time a low specific consumption of electrical energy can be achieved.

We claim:

1. A method of operating a cell of at least 50 ka. for the production of aluminium by electrolysis of aluminium oxide in a molten fluoride bath with pre-baked carbon anodes, comprising operating the cell with that anodic current density j selected according to FIG. 2 with which, with an electrolyte temperature between 940 and 975 C., with an interpolar distance of to 6 cms., and with an aluminium oxide covering of about 14 to 16 cms. thickness on the encrusted surface of the bath, as much heat is produced in the cell as the cell can carry away as losses after deduction of the useful amounts of heat for the decomposition of the aluminium oxide and for heating of the raw materials.

2. A method of operating a cell of at least ka. for the production of aluminium by electrolysis of aluminium oxide in a 940 to 975 C. hot molten fluoride bath with pre-baked carbon anodes, comprising operating the cell simultaneously with an interpolar distance of 5 to 6 cms., with an aluminium oxide covering of about 14 to 16 cms. thickness on the encrusted surface of the bath and with that anodic current density 1' selected according to FIG. 2 with which as much heat is produced in the cell as the cell can carry away as losses after deduction of the useful amounts of heat for the decomposition of the aluminium oxide and for heating of the raw materials.

3. A method of operating a cell of at least 50 ka. for the production of aluminium by electrolysis of aluminium oxide in a molten fluoride bath with pre-baked carbon anodes at a temperature between 940 and 975 0., comprising maintaining an interpolar distance of 5 to 6 cms., an aluminium oxide covering of about 14 to 16 cms. thickness on the encrusted surface of the bath and that anodic current density j selected according to FIG. 2 with which as much heat is produced in the cell as the cell can carry away as losses after deduction of the useful amounts of heat for the decomposition of the aluminium oxide and for heating of the raw materials.

References Cited UNITED STATES PATENTS 3,632,488 1/1972 Decker et a1. 204-228 X 3,575,827 4/1971 Johnson 204-67 3,607,685 9/1971 Johnson 204-67 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner 

